Integrated services digital network is defined by internationally accepted standard digital network user interfaces. The resulting network of access lines and switches is a general purpose digital network capable of supporting access from terminals to a wide range of interconnected services, such as, voice, data, facsimile, and video, by using the standard digital network-user interfaces, i.e., the interfaces between the network and the users. Currently there are two CCITT recommended standard integrated services digital network interfaces for user access. They include the basic rate interface (BRI) and a primary rate interface (PRI).
A variety of voice and data services are available through the integrated services digital network, basic rate interface. Known technology allows the loop plant to transport voice, data, and out-of-band signaling through a four-wire bus, a network termination, and a two-wire loop to a central office. Packet switching in the network provides economical data switching by freeing common resources, during nonactive periods in one call, for use by another call. The International Telegraph and Telephone Consultative Committee (CCITT) defined a Q.931 out-of-band signaling protocol for call control from and to end terminals in an integrated services digital network.
The central office switch provides an integrated services digital network, basic rate interface including two sixty-four kbps B-channels B-1 and B-2 used by voice and/or data calls and one D-channel used for signaling and/or low speed data. A B-channel is a bearer channel. A D-channel is a demand channel. With the integrated services digital network, basic rate interface, the switch supports both the CCITT standard four-wire, T digital subscriber loop interface and the American National Standard Institute (ANSI) standard two-wire U interface. The basic rate interface is arranged to provide simultaneous voice and data services in several ways giving users flexibility in configuring their services. The users use either B-channel for voice service, for circuit switched data transport, or for packet switched data services. The D-channel can carry packet switched data which interleaves data packets with signaling packets. The Q.931 out-of-band signaling protocol is carried out through the D-channel on a time shared basis. Standard NT1 and NT2 network terminations are interposed between the central office and the terminal equipment.
Sixty-four kbps pulse code modulation (PCM) technology presently is used for converting analog voice signals to digitized voice signals which are transmitted through one of the B-channels. In that technology, analog voice signals are sampled at regular intervals. Those samples are quantized into discrete values. The discrete values of the samples are encoded into eight-bit binary code words, and those code words are converted into a sixty-four kbps pulse code modulation bit stream for transmission through a B-channel. The aforementioned prior art sixty-four kbps pulse code modulation bit stream requires an entire sixty-four kbps B-channel for transmission in the known integrated services digital network, basic rate interface. A data B-channel also uses an entire sixty-four kbps B-channel for transmission in the known integrated services digital network, basic rate interface.
As a result of the required channel capacities in the prior art, the basic rate interface has been limited to providing a maximum of either two data B-channels or one voice B-channel and another voice or data B-channel. The prior art limit of one voice and one data channel or the other prior art limit of two voice channels per basic rate interface is insufficient voice channel capacity for providing adequate voice service for some large families or high-income families and for some cottage industries and other small businesses.
A problem arises in a single basic rate interface when two voice terminals or a data terminal and a voice terminal are active simultaneously and a third voice terminal desires to commence operation at once or is being called. With the present arrangement, no B-channel is available for commencing immediate operation by or to the third voice channel.